The invention concerns a resonator arranged on a hollow cylindrical support for the taking of high resolution nuclear magnetic resonance spectra from a sample which extends along the axis of the cylinder within the resonator, having a shielding which defines the region of sensitivity of the resonator along an axial length which is shorter than the axial extent of the support.
A resonator of this type is known in the art as an NMR probe from DE 43 04 871 A1.
In a high resolution nuclear resonance spectrometer, the radio frequency resonator or the radio frequency coil for excitation and detection of nuclear resonance signals from a sample is generally located in a so-called probe head within the axial room temperature bore of a superconducting magnet at the extremely homogeneous center of the static magnetic field produced thereby. In general, the sample to be measured is a liquid within a hollow cylindrical sample tube whose central region is surrounded by the radio frequency coil, which is normally a saddle coil.
It is desirable to limit the region of sensitivity of the radio frequency coil to an axially well-defined length. In order to maintain the full resolution capability the static magnetic field must be as homogeneous as possible along the entire sensitive region. The field homogeneity in high resolution nuclear resonance spectroscopy is 10.sup.-9 and better. In addition to the fundamental inhomogeneity of the magnet, additional field distortions occur due, in particular, to the susceptibilities of the finite length sample and the high frequency coil as well as other weak magnetic components of the probe head. The excitation or detection of nuclear resonance signals from sample regions in which the static magnetic field significantly deviates from its central value should therefore be avoided. Such regions are the end regions of the RF coil as well as regions in which the leads are in proximity to the sample. With the considerably openly structured RF saddle coil, the RF field extends far beyond the central sample region and the RF field lines can close at rather distant sample regions. The above mentioned publication, DE 43 04 871 A1, therefore describes both a hollow cylindrically-shaped shield for the coil end regions and the leads as well as a shield for the RF stray fields effected by two conducting sheets axially above and below the RF saddle coil.
The utilization of a saddle coil with, perhaps, an associated shielding as a radio frequency coil in a high resolution NMR spectrometer has a number of fundamental disadvantages. Due to its open structure it has a poor fill-factor, e.g. only a fraction of the RF magnetic field produced is utilized to excite the sample or only a fraction of the sample signal is detected. For the same reason an RF stray field occurs outside of the actual sample region of interest which excites signals in undesirable sample regions. This stray field can only be shielded with difficulty and not always with a satisfactory degree of success.
A pair of RF saddle coils is a rather asymmetric object which, with non-vanishing magnetic susceptibility of the coil material, produces distortions of the static magnetic field which are difficult to correct. Since one attempts to achieve an increased fill-factor to increase the signal-to-noise-ratio, the coil surrounds the sample in as close a manner as possible and these distortions effect the sample in the sensitive region. A magnetic compensation of the coil wire can provide assistance. However, such compensated wires are extremely difficult to produce. In addition, a residual reduced susceptibility generally remains.
An RF saddle coil pair is only one component of the radio frequency resonant circuit which also includes at least one capacitor. Although same is located within the probe head it is nevertheless spatially separated from the RF coil and connected to same by means of conductors having lengths of several centimeters. This increases the ohmic portion in the resonance circuit and leads to additional undesirable stray inductance.
The RF magnetic field produced by a pair of saddle coils is only moderately homogeneous along the sample.
In order to detect a signal in quadrature it is necessary to utilize two saddle coil pairs which are rotated about the resonator axis by 90.degree. with respect to each other, leading to additional topological problems.
It is therefore the purpose of the present invention to introduce a resonator of the above-mentioned kind for the excitation and detection of nuclear spin resonance signals which produces only easily correctable static field distortions in its region of sensitivety, which has an improved fill-factor and reduced ohmic losses, produces a largely homogeneous RF field, and avoids topological problems with quadrature detection.